1. Technical Field
The present invention relates to a circuit board, and more particularly to a structure of a base for the circuit board and a method of fabricating thereof.
2. Description of Related Art
A flexible circuit board is a circuit board that applies a flexible organic material as a substrate. Moreover, the flexible circuit board can be utilized in products with continuous dynamic bending. Currently, the flexible circuit boards mostly have electrodes fabricated with copper. In order to provide sufficient space to accommodate a glue material required for bonding, the thickness of the copper electrode (or a circuit) usually ranges from 8 μm (micrometer) to 12 μm. However, the copper electrode has a great thickness, a low flexibility, and a large difference in the thermal expansion coefficient with an organic substrate; thus, a stress generated after bonding or during bending may negatively affect the reliability of the circuit board. Moreover, in a process of etching a copper layer to form the copper electrode, since the copper layer is thicker, spaces of etching will be too small under the constraint of an etching angle. As a consequence, a bottom portion of the copper electrode can not be separated completely by etching and will result in short circuit. On the other hand, the over etching will cause an area of a top portion of the electrode to be too small, which affects the conductivity of the electrode. To prevent short circuit from occurring, the copper electrodes usually need to maintain sufficient spaces between one another so that the development of miniaturization is limited.
In addition, patents regarding the flexible circuit boards include the U.S. Pat. No. 7,250,575, for example. The method of the patent is to change the gold bump fabricating process originally at an IC end into a flexible fabricating process. Also, in the U.S. Pat. No. 5,949,512, when the flexible circuit board bends, a mechanism design is used to solve the Young's module difference between the copper and the organic material that causes the stress to concentrate and the copper circuits to break. Furthermore, the U.S. Pat. No. 7,299,547 mainly replaces the original copper circuits with the organic flexible circuits, so that the reliability of the structure is improved.